For the black & white wet on wet - did you not get any black bleeding into the white? I'm guessing not but is there a secret to that! I feel like any time I use black or red I end up with bleeding.
One trick that works for me is to add white food coloring to my base icing, before you color any of it. I am generous with the white. Once I started doing this, I've never had color bleed again. I'm sure there are other tricks, but this works for me.
I always add WHITE food gel to my base icing in the mixer and then make all my colors. This significantly reduces (and even eliminates) issues like color and butter bleed. I also let the white icing 'fall' onto the black. I'm not 'injecting it' or 'dragging it' onto the black base. I find this reduces the fuzziness and color bleed. Finally, icing consistency is key. If your icing is too thin, you'll see some issues with color bleeding. So double check to make sure it is a 10-second icing. For a review on consistencies, check out this blog post with helpful videos - flourbox.com/blogs/tutorials/cookie-decorating-101-the-basics-of-making-dough-and-icing
